Block copolymer (BCP) lithography has emerged as a promising tool to create highly regular and dense dot or line arrays at sub 50 nm length scales. Block copolymer lithography refers to the use of self-assembled domain structures, typically spheres, cylinders, and lamellas in the form of a thin film as a template for the addition and subtraction nanofabrication processes. Two pattern geometries have been studied, dense arrays of dots and dense arrays of lines and spaces. The former can be generated from sphere-forming BCP or from cylinder-forming BCP with domains oriented perpendicular to the substrate, and the latter from cylinder-forming BCP with domains oriented parallel to the substrate or lamella forming BCP with domains oriented vertically to the substrate. Controlling the orientation of the domains on the substrate is critical for useful BCP arrays. Once formed, one of the copolymers is selectively etched away, leaving a transferable pattern. Periodic BCP arrays find use in the fabrication of a variety of microelectronic devices such as magnetic storage media, quantum dot arrays, photonic crystals, photovoltaic cells, and nanowire transistors.
Lamella and cylinders that are oriented perpendicular to the underlying substrate may have advantages in pattern transfer over spheres or parallel cylinders because of the higher aspect ratio of the resulting template and the vertical side-walls. The perpendicular alignment of block copolymer domains in thin films can be controlled by directional fields such as solvent evaporation, electric fields, or directional crystallization. Physical constraints (topography), and chemical patterns can also control the domain orientation. Of these approaches, chemical modification of the substrate is often used to induce perpendicular domain orientation. For example, the substrate can be chemically modified by self assembled monolayer (SAM). SAM of alkylthiols (on gold) or alkylsiloxanes (on Si/SiO2) with polar or non-polar terminal groups have been utilized to modify the substrate. Furthermore, exposure to different doses of X-ray in the presence of oxygen can alter the polarity and hence the wetting behavior of the SAMs resulting in a symmetric, neutral or asymmetric wetting layer.
Random copolymer brushes are also used for chemical modification. Brushes are thin polymer layers in which each chain is chemically grafted to a surface. For example, brushes with hydroxyl groups that attach to silicon oxide surfaces through condensation reactions are a class of surface-modifying layers. Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275 (5305), 1458-1460. In, I.; La, Y. H.; Park, S. M.; Nealey, P. F.; Gopalan, P. Langmuir, 2006, 22, 7855-7860. Although such brushes are useful for modifying hydroxyl-rich surfaces, many substrates such as metals and polymers may not have a sufficient density of surface hydroxyl groups.